In the past, relay valves were used on occasion to ventilate compressed air cylinders and couplings in motor vehicles to shorten the application and response times.
A well known relay valve described in WABCO publication "Description of Compressed Air Devices in Motor Vehicles", April 1978 edition, page 83, Relay valve 973 001, included a fluid pressure inlet, a fluid pressure outlet, a control inlet and an exhaust outlet. A relay piston separates a primary control chamber which is connected with the control inlet from a secondary control chamber connected with the fluid pressure outlet. An inlet chamber which is able to be connected with a source of fluid supply pressure by the fluid pressure inlet in which a multi-way valve is located. By means of this valve, the secondary control chamber either can be supplied with a fluid pressure from the supply source or can be vented to the atmosphere.
The multi-way valve has an operative connection with the relay piston. The relay piston is acted upon either by the control pressure of the control chamber or by the delivery pressure in the secondary control chamber.
When the force of the control pressure in the primary chamber exceeds the force of the delivery pressure in the secondary control chamber, the multi-way valve opens the connection from the inlet chamber to the secondary control chamber so that the air in the receiver is pressurized.
If there is equilibrium between the forces acting on the relay piston, namely, the pressure in the primary control chamber is equal the pressure in the secondary control chamber, the multi-way valve closes and the pressure in the receiver is maintained at a given level. If the pressure in the first control chamber falls, the second control chamber and the receiver are exhausted.
It will be appreciated that relay valves of this type offer the possibility of providing an output pressure which follows the characteristics of the control pressure. In instances where there is a significant difference in the size of the two control or working surfaces of the relay piston which are acted upon by the pressures, it is also possible to vary the controlled pressure though it is higher or lower than the control pressure.
Further, as a rule, relay valves of this type exhibit a linear characteristic. Therefore, such relay valves are not universally usable unless additional special measures are taken in certain applications. For instance, if pressure response changes, and a nonlinear characteristic exists, it is necessary to maintain at a predetermined constant or near constant delivery pressure value when the control pressure only varies slightly. Then, after a predetermined pressure value has been exceeded, the delivery pressure and the control pressure follow a linear characteristic having a predetermined slope and may be handled by the usual relay valves.